1. Field of the Invention
The present invention relates to a high specific-gravity EPDM composition, a dynamic damper composed of the high specific-gravity EPDM composition, a dynamic damper-installed tennis racket, and radioactive ray shielding material composed of the high specific-gravity EPDM composition. More particularly, the high specific-gravity EPDM composition of present invention has improved moldability and weatherability to be used as the dynamic damper that is mounted on sports goods and the like and as the radioactive ray shielding material of radiographic inspection appliances and the like.
2. Description of the Related Art
The dynamic damper (vibration-damping material) is often used to reduce and relieve impacts and vibrations generated in sporting ball-hitting goods and the like when they are used. For example, in the tennis racket, the dynamic damper is fixed to the racket frame thereof to resonate the dynamic damper with vibrations of the racket frame when the tennis racket hits a tennis ball. Thereby the vibrations generated by an impact are relieved to reduce vibrations to be transmitted to a player's hand. In this manner, occurrence of tennis elbow is suppressed.
For example, the present applicant proposed a dynamic damper to be installed on the tennis racket as disclosed in Japanese Patent Application Laid-Open No. 2002-085598 and No. 2002-048185. In the proposal, as the material for the mass-adding part of the dynamic damper, the present applicant proposed a thermoplastic elastomer and chloroprene rubber in which lead and tungsten are dispersed. However, the lead which has been hitherto used in a large amount is inexpensive but the use mode thereof and the use amount thereof are restricted to prevent environmental pollution.
There is a fear that a tennis player touches the dynamic damper or strikes the dynamic damper against others by mistake. In the case where the dynamic damper is formed of a hard material such as metal, there is a possibility that the tennis player is injured, which is not preferable. Ordinary metals have a comparatively low specific gravity. Thus when they are used as the mass-adding part of the dynamic damper, the dynamic damper has a large volume, which disturbs the player during the use of the tennis racket. For appearance, it is preferable that the dynamic damper is small.
Therefore the tennis player desires a material not polluting environment, soft, having a high specific gravity, and having a comparatively high strength so that it is not broken when it drops or strikes against an object. For example, if the mass-adding part of the dynamic damper is sheet-shaped and its thickness is as thin as 0.6 mm to reduce its volume, it is necessary for the material having a high specific gravity to be soft in such an extent that the material flows smoothly in a molding die and have a strength in such an extent that it is not broken, even though it is thin and sheet-shaped. Because the dynamic damper is formed by combining the mass-adding part and the viscoelastic part with each other, it is necessary that the material has adhesion to other materials.
For applications other than the dynamic damper, there is a demand for the development of a material which has a high specific gravity (4-13) and is soft. Thus in recent years, a large number of rubbers and resinous materials having a high specific gravity are proposed.
For example, in Japanese Patent Application Laid-Open No.2000-27331, there is proposed a vibration-damping/sound insulation sheet to which slurry containing a large amount of a high specific gravity filler and an emulsion of rubber is applied
In the field of shielding radioactive rays, in addition to a radioactive ray-shielding protection cloths used for medical purposes, only a predetermined portion is irradiated with a necessary amount of the radioactive rays in radiation therapy and measurement and portion which is not necessary is not irradiated. This is because there is a demand for prevention of destruction of normal cells and prevention of excessive exposure to radiation. As such, a radioactive ray shielding material is used in portions other than the portion to be irradiated with the radioactive rays. In fields other than the medical field, the radioactive ray shielding material is also used to shield the radioactive ray in inspections of food, inspections at a custom house, and techniques of analyzing objects without destroying them.
Heretofore, a material in which lead, a lead compound, a lead alloy or antimony is blended in resin or rubber is generally used for the radioactive ray shielding protection cloths. An acrylic plate or the like has been used for comparatively weak radioactive rays. A tungsten and a plate made of an alloy thereof are also used as the radioactive ray shielding material.
As disclosed in Japanese Patent Application Laid-Open No.8-122492, there is proposed a radioactive ray shielding material made of a resin containing a plasticizer in which tungsten is dispersed.
As disclosed in Japanese Patent Application Laid-Open No.10-153687, there are proposed vulcanized fluororubber and EPDM rubber both containing tungsten dispersed therein and also chloroprene rubber containing tungsten dispersed therein.
However, the method of applying the emulsion to the vibration-damping/sound insulation sheet disclosed in Japanese Patent Application Laid-Open No.2000-27331 is capable of forming a coating film, but is incapable of forming molded products thick or complicated in configuration.
In the radioactive ray shielding material disclosed in Japanese Patent Application Laid-Open No.8-122492, tungsten may settle in the process of drying and removing a solvent. Thus there is a room for improvement in heat resistance and strength of the radioactive ray shielding material.
As described above, as the material for the radioactive ray shielding material, the vibration-damping/sound insulation sheet, the soundproof material, and the like, there is a demand for development of a material which is soft and has a high specific gravity. However, the proposed materials are all hard and difficult to handle and moreover there is a room for improvement in unpollutability, moldability, and processability.
In the radioactive ray shielding field, in the case where lead or its alloy is used, it is necessary to prepare a casting mold to process the lead or its alloy into a predetermined configuration and dissolve the lead or its alloy in the casting mold to cast it. A lead-casting work is very costly because it is necessary to dissolve the lead or its alloy and manufacture the casting mold. Further the dissolving the lead or its alloy causes environment around a work place to deteriorate and affect a human body adversely.
In the case where the radioactive ray shielding material is used for inspection of food or the like, when the radioactive ray shielding material is used in contact with a human body directly or indirectly, there is a fear that the lead which has separated from the radioactive ray shielding material contaminates environment. The melting point of the lead alloy is as low as 80° C. Thus in the case where the radioactive ray shielding material having the lead alloy is used for a medical purpose, a medical appliance or the like having the radioactive ray shielding material cannot be heated at about 100° C. although it is necessary to sterilize it in hot water. Further the radioactive ray shielding material having the lead alloy cannot be used for a pipe or the like of an atomic power plant or the like, because the pipe is required to have heat resistance at 200° C. As described above, in the radioactive ray shielding field, there is a demand for the development of a material which is high in specific gravity, the performance of shielding radioactive rays, strength, moldability, and workability.
Chloroprene in which tungsten is dispersed is a little bad in weatherability. Thus in an outdoor exposure test (sunshine) which is conducted in a strict condition and in a sunshine weatherometer test of exposing an object to ultraviolet rays, tungsten and additives are liable to bloom or whiten.
The vulcanized fluororubber disclosed in Japanese Patent Application Laid-Open No.10-153687 is elastic, soft, strong to some extent, and superior in weatherability, but has a room for improvement in adhesion performance and in a process of combining other materials and the fluororubber with each other. The EPDM rubber also disclosed in Japanese Patent Application Laid-Open No. 10-153687 has rubber elasticity and a high strength, is superior in weatherability, and more adhesive than the fluororubber. Thus the EPDM rubber can be processed in combination with an adhesive agent or other materials, but has a room for improvement in moldability, processability, and weatherability in the case where metal powder having a high specific gravity is dispersed in the EPDM rubber.